craft activity ideas

I help out at a toddler group. we've been open for a year now and are planning a little celebration. We'd like to do a collaborative craft activity to mark the occasion. Something like hand prints that will help form a bigger picture.

At my son's nursery they have big paper tree on the wall and everytime someone leaves, they do a handprint with their name and leaving date on it, laminate it and add it to the tree. You could maybe do leaves in one colour for the original group and different colours for any newer members, so you could keep it going after the years celebration is over.